1. Field of the Invention
The present invention relates to an implantable heart stimulating device with means for detecting an evoked response to delivered pacing pulses.
The invention also relates to a method of operating an implantable heart stimulating device.
2. Description of the Prior Art
Several different implantable devices for stimulating a heart are known. The devices are normally able to sense the electrical activity of the heart. Some implantable devices are able to deliver stimulation pulses to and/or sense the right atrium (in some cases even the left atrium) and also to deliver stimulation pulses to and sense one or both of the left and right ventricles. Devices that are able to deliver stimulation pulses to both the left and right ventricles can be called bi-ventricular pacers. Such devices can be used to treat patients who suffer from different severe cardiac problems, e.g. patients suffering from congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver a sufficient amount of blood to the body. CHF can have different causes. It can for example be caused by a left bundle branch block (LBBB) or a right bundle branch block (RBBB). By using bi-ventricular pacing, the contraction of the ventricles can be controlled in order to improve the ability of the heart to pump blood. The stimulation pulses to the two ventricles can be delivered simultaneously, but it is also known that the stimulation pulses to the two ventricles are delivered with a short time delay (VV) between them in order to optimize the pumping performance of the heart.
U.S. Pat. No. 5,720,768 describes different possible electrode positions in order to stimulate or sense the different chambers of the heart.
U.S. Pat. No. 6,070,100 describes that electrodes may be positioned in both the left and the right atrium as well as in the left and the right ventricle.
In connection with implantable pacers, it is known to detect the capture of the heart, i.e. to detect whether the heart actually reacts on a delivered stimulation pulse. This detection is also called evoked response (ER) detection. If the heart is not captured, it is possible to arrange the pacer to deliver a back-up pulse with a higher pulse energy than the first pulse. It is also possible to increase the pulse energy in future stimulation pulses if capture is not detected. In order to save battery it is important that the stimulation pulses are not delivered with an unnecessarily high energy. In order to determine a suitable pulse energy, it is known to perform an automatic threshold/capture search. By varying the energy of the stimulation pulses, and by detecting whether capture occurs, it is thus possible to find a threshold value for the stimulation pulse energy. Based on the threshold value, a suitable stimulation pulse energy can be determined.
U.S. Pat. No. 6,782,291 describes an implantable cardiac stimulation device that senses evoked responses to applied pacing stimulation pulses. A pulse generator applies the stimulation pacing pulses to the heart in accordance with a pacing configuration. A sensor control selects an evoked response sensing electrode configuration from among a plurality of evoked response sensing electrode configurations in response to the pacing configuration. Signal-to-noise ratios obtained with the various electrode configurations are used to select a best electrode configuration for sensing evoked responses. If no electrode configuration yields an acceptable signal-to-noise ratio, it is recommended that the evoked response sensing be programmed off.
U.S. Pat. No. 6,473,649 describes an implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart and suitable for use during capture verification. The device may operate in an automatic capture verification mode, wherein an electrocardiogram signal of a patient's heart is received and used by the device to determine whether a stimulation pulse evokes a response by the patient's heart. The device suspends the automatic capture verification mode and/or adjusts the detection threshold dependent upon detected and/or measured noise, a determined amplitude of evoked response, a determined modulation in the evoked response, or detected and/or measured artifact.
U.S. Pat. No. 6,975,904 describes the optimization of evoked response detection in an automatic capture detection system employed by an implantable cardiac stimulation device. In one aspect, an automatic capture detection function may be temporarily disabled in order to prevent an inaccurate loss of capture detection while a patient is in the process of changing position, or when the sensed activity level exceeds some threshold value. For example, the ER signal morphology may be undetectable during a shift in position by a patient or when the patient is too active, i.e., where the ER is changing a significant amount. The automatic capture detection function may be temporarily disabled to account for an undetectable ER signal, thus preventing an erroneous loss of capture detection by the automatic capture detection system.
A heart stimulation device sold under the name “Regency” by St Jude Medical (formerly by Pacesetter AB) has a so-called autocapture function that includes automatic verification of capture and adaptation to changing detection thresholds. In this device the autocapture algorithm is temporarily disabled when the simulation threshold is above a certain level (e.g. above 3.9 V). Stimulation is then set to a high output (e.g. 4.5 V). Furthermore, in this device the autocapture algorithm is temporarily disabled when noise is detected and for as long as the noise is sensed.